(1) Field of the Invention
The present invention relates to a controller for a continuously variable transmission that is particularly suitable for use in vehicles.
(2) Description of the Related Art
Vehicles with a continuously variable transmission (CVT) have hitherto been put to practical use and are able to enhance fuel consumption while preventing a shock that is caused in changing vehicle speed.
The continuously variable transmission is designed so that stepless speed ratios are obtained. The speed ratios, as shown in FIG. 6, are produced based on a basic speed ratio RB100 previously set according to a vehicle speed Vs. If the accelerator pedal is depressed, the basic speed ratio RB100 is shifted (see dashed lines) Note that in the case of gasoline engines, throttle opening may be used instead of accelerator pedal position.
When the vehicle speed Vs is being held constant, there are cases where engine output control (e.g., control of throttle opening) interferes with speed ratio control relative to the CVT. The interference between them will be explained in detail in the following example. In this concrete example, it is assumed that engine output is controlled by a cruise control system. The continuously variable transmission is controlled by a CVT controller. The engine is a gasoline engine with an electronically controlled throttle valve.
For instance, the driver sets a target speed to a speed of 60 km/h. In this case, when the vehicle is traveling on a level road at a constant speed, the cruise control system controls the throttle valve so that it is opened according to the target speed.
Thereafter, if the vehicle begins to go up a slope, the vehicle speed will be reduced. Because of this, the cruise control system opens the throttle valve wider in order to maintain a speed of 60 km/h.
As a result, the vehicle speed reaches a speed of 60 km/h, but since the throttle valve is opened wider, the CVT controller controls the continuously variable transmission so that the speed ratio becomes higher (see a dashed arrow in FIG. 6). Because of this, the driving torque will increase and therefore the vehicle speed will exceed a speed of 60 km/h.
Therefore, the cruise control system reduces the throttle opening to maintain a speed of 60 km/h which is the target vehicle speed. The reduction in the throttle opening then causes the CVT controller to control the continuously variable transmission so that the speed ratio becomes lower. As a result, the driving torque will be reduced and the vehicle speed cannot maintain a speed of 60 km/h.
Japanese Laid-Open Patent Publication No. hei 11-314536 discloses a technique for preventing the engine output control from interfering with the speed ratio control relative to the CVT during travel at a constant speed.
The technique shown in the aforementioned Japanese Laid-Open Patent Publication, however, requires mathematical modeling of a vehicle and a control program therefor, which are fairly difficult and time-consuming. Moreover, fine adjustments to them involve considerable time, labor, and costs.